1. Field of the Invention
The present invention relates to a switching regulator, and, more specifically, relates to a PWM drive switching regulator driven by a battery power source in which reduction of PWM drive capacity at the time of power source voltage drop is suppressed and a predetermined duty ratio at the time of power source voltage drop is ensured as well as even if the power source voltage drops to a certain degree, a voltage stabilizing control with respect to a variation of an output voltage can be performed.
2. Background Art
Conventionally, for a portable type audio device and a portable type electronic device such as a personal computer, a PHS, a portable telephone set and a PDA, in order to efficiently covert electric power and to obtain a predetermined power source voltage a DC/DC converter making use of a switching regulator as been utilized.
FIG. 3 shows an example of these sorts of witching regulators (DC/DC Converter).
Numeral 10 is a switching regulator, 11 an error amplifier (Err) thereof, 12 a reference voltage generating circuit, 13 a PWM pulse generating circuit, 14 a driver. 15 is a switching circuit in a form of series circuit of a P channel MOSFET transistor Q and a Schottky diode D which is provided between a power source line +Vcc (DC power source voltage at the input side) and the ground GND.
16 is an output terminal thereof, and between the output terminal 16 and the ground GND a power use capacitor C is provided and a coil L is connected between the junction of the transistor Q and the Schottky diode D and the output terminal 16. Herein, as the coil L a coil having a reactance, for example, of about 10 xcexcH is used and as the capacitor C a capacitor having a capacitance, for example, of about 150 xcexcF is used. Further, between the output terminal 16 and the ground GND a resistance type voltage dividing circuit 17 used for output voltage detection is provided, and voltage Vs detected by the resistance type voltage dividing circuit 17 is fed back to the error amplifier 11. The detection voltage Vs is compared at the error amplifier 11 with a comparison reference voltage Vref of the reference voltage generating circuit 12, and an error voltage (error detection signal) Ve depending on the comparison result is inputted to the PWM pulse generating circuit 3.
The resistance type voltage dividing circuit 17 used for the output voltage detection is provided with a series circuit of resistors R1 and R2 and in parallel therewith a speed-up circuit (a circuit for shortening time for moving into an operating condition from a start). The speed-up circuit is constituted by a CR time constant circuit 17a used for gain setting.
The error amplifier circuit (Err) 11 includes a phase correction circuit 18 which is constituted by a series circuit of a capacitor C1 and a resistor R3 and a capacitor C2 provided in parallel with the series circuit, and of which circuit is provided between the output and one of the input thereof as a feedback circuit, thereby, the oscillation of the circuit is prevented, when a PWM drive gain rises.
The PWM pulse generating circuit 13 compares the waveform of a triangular wave generating circuit 13b at a comparator 13a with the error voltage (the voltage depending on the comparison result) Ve and produces a PWM pulse by slicing the triangular wave with the error voltage Ve. The PWM pulse is then applied to the driver 14. The driver 14 turns ON/OFF the transistor Q depending on the pulse width thereof and generates a stepped down voltage (in case of a stepped up type, a stepped up voltage by means of a fly-back pulse) at the output terminal 16.
The Schottky diode D is a fly wheel diode to which the current flown out from the coil L when the transistor Q is turned OFF is commutated.
Thereby, in the switching regulator 10, the transistor Q is ON/OFF controlled so that the voltage divided by the resistance type voltage dividing circuit 17 coincides with the comparison reference voltage Vref and the output voltage generated at the output terminal 16 is controlled to assume a constant voltage Vo representing a target, and thus the output voltage is stabilized.
Further, as the power source (Vin=power source voltage+Vcc) at the input side which is connected to the power source line +Vcc as a power supply source a battery as shown by dotted lines is usually used.
However, in such circuit, although the amplitude xcex94V (see FIG. 4) of the triangular wave of the triangular wave generating circuit 13a varies in the direction in which the amplitude slightly reduces in response to a power source voltage drop, however, with this measure the stabilizing control at the time of power source voltage drop can not be performed. Since the phase correction by the phase compensation circuit is set in correspondence with the power source voltage under a normal condition, such a PWM control is required in which the ON period of an output transistor is, for example, set at more than 80% so as to stabilize the output voltage at the time of power source voltage drop.
When performing such PWM control, as seen from a relationship between the waveform of the triangular wave and the error voltage Ve in the PWM control as shown in FIG. 4, the duty ratio of the PWM drive pulse rises and a control is performed near at the bottom of the triangular wave T. For this reason, the variation range of the pulse width with respect to the variation of the error voltage Ve is suppressed and limited and causes a problem which prevents a control in response to an output voltage variation at the time of power source voltage drop.
Further, in FIG. 4 abscissa represents amplitude voltage value of the triangular wave and ordinate represents time t.
An object of the present invention is to resolve the above conventional art problems and to provide a switching regulator in which reduction of PWM drive capacity at the time of power source voltage drop of the switching regulator is suppressed and a predetermined duty ratio at the time of power source voltage drop is ensured as well as even if the power source voltage drops to a certain degree, a voltage stabilizing control with respect to a variation of an output voltage can be performed.
A switching regulator of the present which achieves the above object and which receives an electric power from a DC power source having a predetermined power source voltage, generates an error voltage depending on difference between an output voltage and a target voltage, produces a PWM drive pulse through comparison of the error voltage with a triangular wave having a specific frequency from a triangular wave generating circuit and controls the output voltage so as to assume the target voltage, by switching a transistor with the PWM drive pulse, is characterized in that the triangular wave generating circuit includes a capacitor for charging and discharging; an amplitude voltage generating circuit which generates a triangular wave amplitude voltage in response to the power source voltage; a charging and discharging current producing circuit which produces current having a current value corresponding to the amplitude voltage as a charging and discharging current for the capacitor and a charging and discharging control circuit which receives the amplitude voltage and changes over the current from the charging and discharging current producing circuit from charging to discharging or from discharging to charging according to the amplitude voltage, and generates the triangular wave having the specific frequency as a terminal voltage of the capacitor.
According to the above structure, the amplitude of the triangular wave from the triangular wave generating circuit is controlled so as to be reduced depending on the drop of the power source voltage and further the current value for charging and discharging the capacitor is also controlled so as to be reduced depending on the amplitude. Through maintaining the frequency of the triangular wave to be outputted at a predetermined frequency by reducing the current value for charging and discharging together with the amplitude when the power source voltage drops, the gradient of the triangular waveform can be reduced. Through the reduction of the gradient of the waveform of the triangular wave, the error voltage is shifted from the bottom portion of the voltage of the triangular wave toward the center side thereof and the required comparison is performed.
Further, although a battery is usually used as the power source, however, the power source is not limited thereto and in the DC power source generally the power source voltage is substantially the constant. Accordingly, the voltage amplitude of the triangular wave generated in response to the power source voltage is kept substantially constant. The charging and discharging current for the capacitor in response to the voltage amplitude also assumes substantially constant current. Therefore, the generated triangular wave takes a waveform having a linear gradient.
Accordingly, a variation range of the PWM pulse width can be enlarged with respect to the variation of the error voltage and a sufficient control capacity can be achieved with respect to the output voltage variation when the power source voltage drops.
As a result, a reduction of PWM drive capacity can be suppressed when the power source voltage drops in the switching regulator which is driven by a battery power source, a predetermined duty ratio can be ensured even at the time when the power source voltage drops, and a PWM drive can be realized sufficiently with respect to the output voltage variation when the power source voltage drops.